The UN children's agency Unicef said it was shocked by the "rapid escalation of grave violations against children".
The Taliban have been making major advances across the country as foreign troops withdraw, taking six regional capitals since Friday.
They have rejected international calls for a ceasefire.
More than 1,000 civilians have been killed as a result of the conflict in the past month.
In a statement on Monday, Unicef said the atrocities being committed against children were growing "higher by the day".
